There are 2 upstream o2 sensors on cadillac sls and 2 downstream. There are 2 exhaust manifolds on the 8 cyl 4.6 L bank 1 sensor 1 which is backside manifold and bank 1 sensor 2 which is front manifol

You have 2 of those and also 2 downstream. You have two banks of cylinders and each one has an upstream o2 sensor and it is located in the exhaust manifold for each bank. Your scan tool should say Bank 1 or Bank 2.

The downstream sensors are after the catalytic converter.

What are the symptoms of a bad upstream O2 sensor? Sensor 1 Sensor 1 is the upstream oxygen sensor. It is the sensor that measures the oxygen content in the exhaust, providing an input to the computer, which determines how to adjust the air/fuel ratio. Sensor 1 is the sensor closest to the engine. Sensor 2 is the downstream oxygen sensor.
Mar 10, 2021

Bank 1, Sensor 1? Upstream or Downstream? - Newparts.com

https://www.google.com/search?q=2015+Chevy+Impala+3.6+upstream+o2+sensor+location

There are 3 O2 sensors on the chevy vortec V6. 1 upstream left bank, 1 upstream right bank, 1 downstream.

Downstream sensor is located at outlet side catalytic converter. The two upstream sensors are in the left and right exhaust before they join. Look behind front wheels.

On most vehicles, the sensors are screwed in to the exhaust pipes or exhaust manifolds, usually under the car/truck , in the vicinity of "under the front seat" area. They are usually the size of your thumb, and have a wire going to them.

You did not say what your car was ,however most newer cars use 2 oxygen sensors per bank, if it is a 4 cyl replace downstream o2 sensor,if 6 or 8 cyl bank 1 sensor 2 is downstream sensor on bank that has cyl 1 on it.upstream sensor is in front of catalytic converter downstream is after the cat

4 total. 1 each manifold. Then 1 before, 1 after the CAT. You should be able to count and trace them from the connections on the top of your engine. Good luck

Here is the low-down on oxygen sensors, and their location designations explained:

Oxygen sensors are screwed into the exhaust system at various points. Some people say that there are 4 oxygen sensors, but technically, that is not correct. Only the front 2 are considered oxygen sensors.

These oxygen sensors are screwed into the exhaust pipe or directly into the exhaust manifold, one on each bank. Because they are in front of the catalytic converter, this is often referred to as 'upstream'.

The other 2 are actually called catalyst monitors. They are oxygen sensors, but they serve a different purpose. They monitor the activities of the catalytic converters, thus the name, catalyst monitors. They are screwed into the exhaust one on each bank, after, or downstream, of the catalytic converters.

On a 4 cyl engine, there are only a total of two such sensors, the one in front of the catalytic converter is the O2 sensor, the one behind the cat is the catalyst monitor.

The location names for them are as follows:

• On a 4 cyl, the front / upstream is called "bank 1 sensor 1" and the rear / downstream sensor is called "bank 1, sensor 2."
  
  
• On a V6 or V8 engine:
  
  
  • the passenger side bank on a forward facing engine, or the bank closest to the firewall on a transverse mounted engine, is bank number 1, so the forward / upstream sensor on that bank would be "bank 1, sensor 1."
    
    
  • The driver's side bank (forward facing mount engine), or front bank (transverse engine) would be bank number 2, thus the forward / upstream sensor on that bank would be "bank 2, sensor 1," and the rearmost / downstream sensor on that bank would be "bank 2, sensor 2."

The answer depends on the type of exhaust system you have. You will have either 3 or 4 total O2 sensors on your V8. If you have single exhaust with a single catalytic converter then you will have 3 O2 sensors (2 upstream sensors & one downstream sensor - "up or down stream" meaning before of after the catalytic converter). If you have either dual exhaust and/or two catalytic converters then you have 4 O2 sensors (2 upstream & 2 downstream). If you have an MIL light (malfunction indicator light, or "check engine light") the code will indicate "front bank / bank one", which is one of the 2 upstream O2's before the catalytic converter(s), or "rear bank / bank two", which is one of the O2's after the catalytic converter(s).

If the engine is a V6 (2.7 or 3.5 lit), bank 1 is next to the firewall (right hand) when facing the back of the engine.
If the engine is a 4 cly (I4), there is only one bank and is designated Bank 1 (exhaust manifold).

The catalyst to which the code refers is the UPSTREAM, and there is only ONE upstream cat for any bank.

Clue: Two upstream O2 sensor and two downstream O2 sensors - one before the CAT and one after the CAT. the downstream O2 sensors are identical to their upstream sisters. The only reason for the DOWNstream sensor is to monitor the CAT efficiency (threshold).

The catalyst on either bank is essentially the exhaust manifold which houses the CAT... it is the only CAT to which the code P0421 applies.

Hope that helped

Oxygen sensors are always numbered like this:
Bank 1 sensor 1
Bank 2 sensor 1
Bank 1 sensor 2
Bank 2 sensor 2

Some manufacturers use a kind of shorthand that reads different, but means the same thing:
Sensor 1/1 or O2s 1/1
Sensor 2/1 or O2s 2/1
Sensor 1/2 or O2s 1/2
Sensor 2/2 or O2s 2/2

Bank 1 is always the side of the engine where cylinder number 1 is located and, of coarse, Bank 2 is the opposite side.
On a 4 cylinder engine, there is only 1 bank and it is always referred to as Bank 1.

Sensor 1 is always the upstream sensor (The one located BEFORE the catalytic converter)
Sensor 2 is always the downstream sensor (the one that is located AFTER the catalytic converter.

Hope helps (remember to rate this).

Well, Bank 1 is the passenger side of the vehicle. Are you replacing upstream or downstream 02 sensor?

The upstream is in the exhaust manifold. The downstream is further down in the exhaust pipe. Upstream would be designated by the code HO2S1- B1 and the downstream would be indicated HO2S2-B1.

In either case, you basically unplug the wiring connector, use a 7/8 wrench (or an O2 sensor socket which has a groove cut in the side to allow the wiring to not be pinched) and loosen the sensor and install the new one (with anti-seize on the threads) and plug the connector in.

The upstream sensor is pretty easy- the downstream can be a little more difficult, but is not too bad.

Hope this is helpful!!

Nissan Scope Technician- 10+ years experience in Nissan dealership.